1. Field of the Invention
This invention relates to a method for making strong, sag-resistant structural panels of mineral wool and/or mineral aggregate that may vary from less than 8 to about 20 pounds per cubic foot or more. More particularly, it relates to a rapid, practical process for forming and "flow through" air drying a mineral wool panel continuously on a Fourdrinier wire. These panels may be used as acoustical ceiling tiles, thermal insulating panels, sound absorbing panels, pipe and beam insulation and the like products.
2. Description of the Prior Art
The water felting of dilute aqueous dispersions of mineral wool and lightweight aggregate is known. By such methods, a dispersion of mineral wool, lightweight aggregate, binder and other adjuvants are flowed onto a moving foraminous support wire screen, such as that of an Oliver or Fourdrinier mat forming machine for dewatering, at line speeds of about 10-50 feet per minute. The dispersion dewaters first by gravity and then vacuum suction means; the wet mat is dried over a number of hours in heated convection drying ovens, and the product is cut and optionally top coated, such as with paint, to produce lightweight structural panels such as acoustical ceiling products. Such methods cannot produce low density structural products below about 12 pounds per cubic foot density. A "structural" panel, as used herein, is capable of supporting its own weight without visible sagging, bending, or collapsing when supported only at the edges of the panel as in a suspended ceiling grid.
For many years a premium grade of acoustical ceiling tile has been made by a wet-pulp molded process similar to that described in U.S. Pat. No. 1,769,519. The product is formed by bonding nodules of mineral wool with a viscous cooked starch gel. The wet pulp contains about 83-87% by volume (and about 68-75% by weight) water that is necessary to provide proper forming and acoustical properties to the wet pulp. The wet pulp is "screeded" by passing the pulp under an oscillating bar or rotating endless loop belt and the like. The screeding action, by the friction of the bar or belt against the viscous pulp, disrupts some groups of the nodulated mineral wool on the surface of the pulp, creating crevices and fissures in the surface that permit sound wave penetration and impart a visually pleasing appearance. The screeded pulp is then dried in a convection oven for about 12 hours or more. During drying, the water is removed providing a highly porous product with excellent sound absorbing properties.
U.S. Pat. Nos. 1,996,032 and 1,996,033, describe various compositions and methods of wet forming such molded acoustical tiles from a thick aqueous pulp of nodulated mineral wool.
U.S. Pat. No. 3,510,394 discloses flocculating inorganic kaolin clay in dilute dispersions of mineral fiber. Flocculation to clumps or flocs of the clay with starch grains is effected by adding extremely small amounts of flocculant such as polyacrylamide just before the slurry is dewatered, and the wet mat is baked or fired in addition to conventional drying. Dewatering time is increased by this flocculation treatment.
U.S. Pat. No. 4,613,627 discloses a modified wet pulp process for forming an acoustical ceiling tile wherein the binder is foamed separately from the rest of the solid ingredients. The foamed binder is then combined with an admixture of the other solids, and the admixture is cast, screeded, textured, press molded and dried.
The use of foam to prevent stratification of the various particles in a slurry of mineral wool, aggregate and other solids during the water felting of mineral fiber panels is taught by Guyer et al in U.S. Pat. No. 4,062,721. The foam retains the particles in a space matrix but also increases the water drainage time according to Guyer et al who solve that problem by delaying the foaming of the furnish until after gravity drainage has occurred. Guyer et al teach that more water is removed because the foam reduces the gross porosity of the furnish thus making vacuum dewatering more effective. This means that air is not passing through the furnish but pressing down on it and reducing the porosity still further.
Bryant teaches in U.S. Pat. No. 1,841,785 that a tough coherent skin of paper-like consistency may be created on the lower surface of a foamed mass of cellulose fibers and water on a Fourdrinier wire by subjecting the lower surface momentarily to a vacuum without imparting the suction deeply into the mass so that only the lower surface area is compacted. Further dewatering of the foamed mass occurs under a lesser vacuum so that the fibrous body of the mass is not broken down or compacted. The still wet fibrous body is then dried by passing it through an oven into which hot air is blown at levels above and below the fibrous body. The spongy consistency of the body, except for the tough skin, is thus preserved.
Current water-felted and cast acoustical panels exhibit limited stability under high moisture loads. This undesirable characteristic is associated with the hydrophilic nature of the cellulosic fibers used in many such products or the starch binder used there and in mineral fiber panels. There has been interest in latex resins as binders but their high cost and high loss in the water felting procedure have been discouraging. Moreover, regardless of the type and cost of the binder, two major factors in the cost of producing acoustical structural panels have been the energy and the time expended for the dewatering and drying of the felted fibers.
This problem has been addressed in U.S. Pat. No. 4,587,278 wherein Dotzauer et al teach the use of certain thermoplastic polymers as the binder for mineral fibers in a sound insulating board. The polymer may be added to an aqueous suspension of mineral fibers and then precipitated onto the fibers by the addition of a cationic polymer dispersion or a salt of a polyvalent metal. Dewatering of the suspension is done on a sieve under reduced pressure and with gentle pressing. Infra-red lamps, hot air or microwaves are used to dry the resulting sheet at from 110.degree. to 220.degree. C. Dotzauer et al teach that the migration of the polymer particles during drying may be prevented by adding heat sensitizing agents to the polymer dispersion before the sheet-forming procedure is commenced. The drying time reported is from 36 minutes to 3 hours.
Before Dotzauer et al's use of the thermoplastic polymers as binders, Waggoner taught in U.S. Pat. No. 3,228,825 that cellulosic fibers and inorganic fibers such as asbestos and rock wool may be used as binder fibers in forming glass fibers into a felted mass. The binder fibers cling to the surface of the glass fibers and a mechanical interlocking of the binder fibers causes the felting to occur. They also function as spacers to separate the glass fibers in the felted mass. When used as taught, the binder fibers do not cause a problem in the elimination of the water; it is possible to suck hot air through the deposited fibrous structure so that drying can be achieved in a short time.
Now, it has been discovered that a latex binder may be coupled to the mineral fibers as the sole binder in an aqueous slurry to form a wet open mass of entangled fibers so strong that it does not collapse when air is blown through it to achieve rapid drying.
It is an object and advantage of the present invention, therefore, to provide a method for manufacturing low density, structural panels by the wet felting of mineral fibers without having to dry extremely large amounts of water out of the wet mass over long periods of time.
It is another object of this invention therefore, to provide a self sustaining, highly voided, wet mass of entangled mineral fibers which withstands the force of high velocity streams of air rushing through it.
A further object is to provide strong mineral fiber panels having densities between 3-10 pounds per cubic foot and a modulus of rupture at least about 30 pounds per square inch for the core of the panel.
Another object is to provide mineral fiber and/or lightweight mineral aggregate structural panels which have excellent strength and integrity at densities up to about 22 pounds per cubic foot or more.
Another object and advantage is to provide a low cost method and a composition for manufacturing rigid acoustical ceiling title which exhibits good strength and little if any sag or warpage in dry or humid conditions.
A still further object and advantage is the provision of a practical method for manufacturing lightweight mineral fiber panels wherein the dewatering and drying of a latex resin bonded mat may be accomplished in a facile, rapid manner such that the mat is dewatered and dried in as little as 10 minutes.
The above objects and advantages, and others which will become more apparent from the drawings and the ensuing description, are accomplished by forming a dilute aqueous dispersion furnish of mineral fiber and/or mineral aggregate and an anionically-stabilized resin latex binder, such as a polyvinyl acetate. Almost at the end of mixing, a small but effective amount of a flocculant, such as a cationic polyacrylamide, is added and the furnish is passed to a flooded section of a drainage wire of a foraminous mat forming apparatus. In this system and at the levels added, the flocculant does not cause clumping of the latex particles with each other. Rather, it acts as a coupling agent, the latex particles being dispersed throughout the water and coated upon the mineral materials. Virtually all of the binder resin solids added to the system become coupled to the mineral surfaces in the wet felted product, with very low losses of binder resin in the drainage section white water. Thus, the drainage water need not be recycled to avoid substantial losses of binder solids. The mat contains almost half the total solids of a cast pulp but may be dried in a matter of minutes rather than many hours by passing large volumes of heated air through it. By adding various proportions of aggregate in this particular method of felting, water stripping and drying of the panel products as open, porous structures may be performed at various densities, ranging from about 3 to about 22 pounds per cubic foot or more.